Luminescent materials or phosphors comprise a host material which together with one or more activators converts absorbed energy into radiant energy. Some of the most widely used host materials are calcium halophosphate, barium magnesium aluminate, magnesium aluminate, strontium chloroapatite, zinc silicate and the oxides, oxysulfides, phosphates, vanadates and silicates of yttrium, gadolinium or lanthanum. Commonly used activators are rare-earth ions such as europium II and III, terbium III, cerium III, and tin II. Conventionally, compounds containing the host and activator materials are mixed together by a mechanical procedure, such as milling or micronizing, and then calcined to produce fine powder phosphors of relatively uniform composition. These powders may be bonded as coatings to the surface of a substrate, typically glass, for use in a variety of applications.
Rare-earth oxide (REO) activated phosphors are used in trichromatic lamps, cathode ray tubes (CRT), color television (CTV), high definition television (HDTV) screens, among others. Many of the rare-earth oxides are very expensive and this has limited their use in a number of applications. For example, conventional three-wavelength fluorescent lamps emit white natural light by correctly blending three narrow spectral bands of blue-violet, red-orange and green. Europium II-activated barium magnesium aluminate or europium II-activated strontium chloroapatite can be used as the blue phosphor, europium III-activated yttrium oxide can be used as the red phosphor and cerium/terbium-activated magnesium aluminate can be used as the green phosphor. Such fluorescent lamps have greater color rendition, a longer useful life, and are more energy efficient in comparison to incandescent lamps. The marketability of these fluorescent lamps has been hindered, however, because of their high cost which is attributable to the necessity of using expensive rare-earth oxides.
Czechoslovakian Patent No. 266,856 B1, to Hajek et al., claims a luminescent composition comprising a silica core, on the surface of which is coated a luminescent layer of mixed rare-earth element oxides. Hajek et al, purport that this achieves a reduction in the amount of expensive rare-earth phosphors used while the required light parameters are retained. These compositions are prepared by using a rare-earth salt solution/oxalic acid diethyl ester system to precipitate rare-earth element oxalates onto the surface of activated silica particles, and subsequently calcining the oxalates to obtain an oxide luminophor. The examples in this patent describe coated silica particles in which the rare-earth phosphor layer is between about 60 and 95 wt % of the composition. At the coating thicknesses described in the examples of this patent, only a modest, if any, reduction in the consumption of expensive rare earth elements is achieved.
The disclosure of the above-identified Patent is hereby incorporated by reference.